Sauron had been contending with the Elves for mastery of Middle-earth for over 1200 years. Fearing the establishment of the powerful kingdoms in exile — Arnor and Gondor — by his hated enemies, the Dúnedain, Sauron launched a pre-emptive attack on Gondor in S.A.3429.

In response, Elendil, High King of the Dúnedain, formed an alliance with Gil-galad, the High King of the Noldor, to repel Sauron's assault. Durin's Folk also aided Elendil and Gil-galad. It is said that Elendil bound the Alliance with an oath and invoked the name of Eru to witness it.[1] Eru's name was very rarely invoked in oaths, and among the Numenoreans it was held that only the King could call upon Him. This would happen again only thousands of years later during the Oath of Cirion.[1]

The hosts of Gil-galad and Elendil met at Amon Sûl and marched towards Imladris where they camped for three years, forging armour and making plans whilst Anárion, Elendil's younger son, defended Osgiliath against the hosts of Mordor.

In S.A.3434 they defeated Sauron's army in the Battle of Dagorlad, breached the Morannon into Mordor, and besieged the Dark Lord's fortress of Barad-dûr. The siege lasted for seven years, during which Anárion was slain. It culminated in Sauron leaving his fortress and engaging in direct combat. There were three objectives to this war; to unmake the One Ring, to destroy Sauron, and to destroy the foundation of the Dark Tower.

Sauron was felled by Gil-galad and Elendil, who both perished in the assault themselves. Elendil broke his sword Narsil as he fell. Using the hilt-shard of the sword, Isildur cut the Ring from Sauron's finger. Bereft of the power of the One Ring, Sauron's spirit dissipated and would not take form again in Middle-earth for a thousand years.

With the victory over Sauron, and the death of Gil-galad and Elendil, the Last Alliance was dissolved. The remaining Númenóreans resettled their kingdoms.